This invention relates to a process for removing dust, sulfur compounds and nitrogen oxides from combustion exhaust gases and to an apparatus for carrying out the process.
In the generation of electrical power by combustion processes and for carrying out endothermic combustion processes using atmospheric oxygen, combustion exhaust gases are produced which contain nitrogen, carbon dioxide, water and oxygen as well as various pollutants. The oxygen content of the combustion exhaust gases amounts to 1 to 10%, as a rule. Pollutants which are regarded as particularly significant include HCl, oxides of sulfur and of nitrogen, i.e. SO.sub.2, SO.sub.3, NO and NO.sub.2, as well as dustlike fly ash, which is entrained by the exhaust gas stream from the combustion chamber. These pollutants must substantially completely be removed from the combustion exhaust gases. The combustion exhaust gases have, as a rule, an SO.sub.2 content between 500 and 10,000 mg/sm.sup.3 (sm.sup.3 =standard cubic meter, and their SO.sub.3 content amounts only to up to 50 mg/sm.sup.3. The content of the oxides of nitrogen in the combustion exhaust gases amounts to about 100 to 3000 mg/sm.sup.3, calculated as NO.sub.2. The dust content of the combustion exhaust gases is about 1 to 10 g/mg/sm.sup.3, as a rule, but may be as high as 50 g/sm.sup.3. In addition to these pollutants present in particularly significant amounts, the combustion exhaust gases contain further pollutants, such as N.sub.2 O, HF, heavy metals, heavy metal compounds, dioxins, furans, highly condensated aromatic hydrocarbons and CO, the removal of which need not be discussed at this juncture.
Known processes for dedusting combustion exhaust gases are carried out on a large scale in practice, and centrifugal separation, filtration and electrostatic precipitation are mainly employed for that purpose. The desulfurization of the combustion exhaust gases, i.e., for a removal of SO.sub.2 and SO.sub.3, is presently effected by wet or quasi/dry/or dry separating processes, in which either aqueous alkaline solutions or solid absorbents consisting of CaO and/or Ca(OH).sub.2 are employed in most cases. In the known desulfurizing processes a major part of the HCl is also removed from the combustion exhaust gases. There removal of the nitrogen oxides NO and NO.sub.2 is presently effected by a catalytic reduction with NH.sub.3 at temperatures from 300.degree. to 500.degree. C. or by a non-catalytic reduction with NH.sub.3 at 750.degree. to 1000.degree. C. A simultaneous desulfurization and removal of nitrogen oxides from combustion exhaust gases has also been proposed.
Published German Application 3,642,980 discloses the catalytic reduction of NO contained in a gas with NH.sub.3 as a reducing agent in a process, in which the NO-containing gas is mixed with NH.sub.3, and the mixture is reacted at 185.degree. to 500.degree. C. and under normal pressure on a catalyst, which consists of an acid support and the catalytically active substances CuSO.sub.4, MnSO.sub.4, FeSO.sub.4 and/or Fe.sub.2 (SO.sub.4).sub.3. In Published German Application 3,642,980 it has also been proposed to effect the catalytic reduction of NO in a fluidized bed, particularly in a circulating fluidized bed. That known process is directed not only to the removal of nitrogen oxides from combustion exhaust gases but permits a deduster to be used, which succeeds the catalytic removal of nitrogen oxides and removes the dust still contained in the exhaust gases.
Besides, Published German Application 3,701,527 proposes a simultaneous removal of nitrogen oxides and sulfur compounds from an oxygen-containing exhaust gas in a process in which the exhaust gas is mixed with NH.sub.3 and is then supplied as a fluidizing gas to a fluidized bed, which consists of a catalyst, which effects a reduction of NO with NH.sub.3 to N.sub.2 and H.sub.2 O and an oxidation of SO.sub.2 with O.sub.2 to SO.sub.3. The catalyst proposed in Published German Application 3,701,527 comprises a carrier, the catalytically active substances V.sub.2 O.sub.5 and Fe.sub.2 O.sub.3, FeSO.sub.4 and/or Fe.sub.2 (SO.sub.4).sub.3 and alkali sulfates. The SO.sub.3 formed by oxidation may be recovered as sulfuric acid or may be bound by a desulfurizing agent, which consists of Na.sub.2 CO.sub.3, MgO, MgCO.sub.3, CaC, CaCO.sub.3, and/or Ca(OH).sub.2. The desulfurizing agent may either directly be supplied to the fluidized bed or may be contacted with the fluidizing gas when the latter has left the fluidized bed. The known process is carried out at a temperature from 350.degree. to 600.degree. C. Whereas the process known from Published German Application 3,701,527 permits a simultaneous desulfurization and removal of nitrogen oxides from combustion exhaust gases, it has the disadvantage that a catalyst is required, for which production costs must be incurred and which is consumed in operation by mechanical abrasion. Besides, the process may be disturbed by a relatively high dust content of the raw gas. Finally, it is extremely difficult to completely remove the unconsumed desulfurizing agent--particularly CaO and Ca(OH).sub.2 --from the gas which has left the fluidized bed.